Methods for filling surface parts of an image with a surface pattern using a raster image memory are well known. They can be used, for example, in an electronic printing system for printing a page of text, a page of pictures, or a page having a combination of text and pictures in response to the user's instructions. The information to be printed can be input by means of a data-processing system through the associated peripherals. It can then be transmitted to a raster output printer by means of a front-end system containing a raster image memory. The raster output printer, such as a laser printer, is used to print, line by line, the data called serially from the raster image memory.
The data relating to a complete page which is to be printed can be stored in the raster image memory. The page is divided up into a large number of raster points or pixels which are present in the form of a matrix. Typically, there is a 1:1 ratio between the raster points and the 1-bit memory places of the raster image memory. Therefore, the raster image memory is also known as a bit map. The memory places of the raster image memory are frequently combined into multi-bit words such as 16-bit words. In such a case the memory is termed a word-oriented bit-map memory.
U.S. Pat. No. 4,300,206 describes a raster image printer in which data of any size and orientation can be printed on a page. A microprocessor receives a succession of coded character and control data. The coded character data are processed by the microprocessor one-at-a-time in accordance with the control data and translated into character bit-map data and after that the microprocessor positions these data of character bit map into a strip buffer. However, because the described system operates in a sequential manner, it is a relatively slow system.
Applicants' Netherlands patent application No. 8503461 filed on Dec. 17, 1985, and the corresponding U.S. application Ser. No. 943,255, describe a high speed method for generating line parts for a raster output scanner which can be used to print both text information and graphic information. The characters or fonts required for the text information are stored in the form of small bit-maps and can be transferred with appropriate instructions to the required places in the bit-map memory and in the required size for the page to be printed.
Specific programs have been created by others for inputting graphic information. These known programs enable graphic elements to be generated such as straight lines, portions of arcs of a circle and the like. At the user's discretion, these various graphic elements and symbols can be combined in various ways by combining the bit-representations of the graphic elements in the bit-map memory by means of suitable logic operations. The contents of the raster image memory are displayed on a screen so that the user can vary the layout of the page to be printed as necessary.
The following references are of general interest. European Patent Application 0174809 discloses a method and apparatus for filling predetermined areas of a graphics display with a predetermined repetitive pattern by using the undisplayed portion of the frame buffer memory to store the replicated patterns. European Patent Application 0158902 discloses a method of transforming color images into monochromatic images.
Often, it is necessary to stress certain surface parts of the image to be printed. This is done optically by filling those parts with a suitable surface pattern such as cross-hatching, dots or lines so as to draw attention to that surface part. When it is necessary to use the same surface pattern for filling a number of surface parts which directly adjoin or overlap one another, special care must be taken to ensure that the surface pattern does not shift at the boundary between the two surface parts or appear double in the overlap zone of the surface parts.
Accordingly, it is an object of the present invention to provide an efficient method for filling surface parts of an image with a surface pattern wherein adjoining or overlapping surface parts have a continuous pattern.